The present invention relates to an SOI (Si-on-insulator) substrate which has a semiconductor layer formed on an insulator and is a promising candidate for the substrate material of the next generation LSI, and a method for manufacturing the same.
Methods of forming the SOI structure which has a semiconductor active layer on an insulator include, for example, the SIMOX (separation by implanted oxygen) process described in the Journal of Material Research, vol.8, No.3, pp.523-534, 1993. In the SIMOX process, oxygen ions (O+) are implanted in a silicon substrate (Si substrate) followed by heat treatment at a high temperature, so that an oxide film (SiO2 film) which continues in a direction parallel to the substrate surface is formed inside of the substrate, with the center of the film located at a depth in the substrate where the oxygen concentration produced by the oxygen ion implantation is maximum (density peak). The density peak is formed at a position of the mean range path (Rp) of accelerated oxygen ions.
With the SIMOX process, although an SOI substrate can be obtained relatively easily, there is such a drawback that a number of crystal defects (dislocation, stacking fault) remain at the position of damage peak, where the damage generated during the oxygen ion implanation is maximum, in the Si active layer which is the region where devices are formed. The damage peak is formed at a depth where the oxygen ion with the given acceleration energy has the highest probability to collide with an Si atom, and usually the depth is three fourths (xc2xe) of the depth of the density peak.
In order to reduce the crystal defects generated at the position of damage peak, for example, Japanese Patent Laid-Open Publication No.Hei. 10-79355 proposes a technology of replacing the damage peak with an SiO2 film by carrying out oxygen ion implantation and high temperature heat treatment processes followed by oxidation at a high temperature. With this technology, the damage layer is absorbed by increasing the thickness of a surface oxide film or a buried oxide film. However, since the buried SiO2 film is not formed directly at the position of damage peak, it is difficult to control and there is such a problem that crystal defects remain in the SOI active region which is interposed between the surface oxide film and the buried oxide film.
As described above, in spite of the advantage that the SOI substrate can be obtained relatively easily, the SIMOX process has the drawback that the crystal defects remain in the upper Si active layer which is the region where the device is formed. Also there is such a problem that it is difficult to completely eliminate the crystal defects even when the high-temperature oxidation is carried out for the reduction of the crystal defects.
In view of the foregoing, an object of the present invention is to solve the problems of the SIMOX process described above and provide an SOI substrate that includes less crystal defects in the Si active layer and a method for manufacturing the same.
In order to achieve the object described above, the SOI substrate of the present invention has a buried SiO2 film which continues in a direction parallel to the main surface of a silicon substrate and a plurality of SiO2 islands disposed below the buried SiO2 film, while being arranged in a direction substantially parallel to the main surface. Or, alternatively, a second SiO2 film which continues in the direction parallel to the main surface of the substrate may be formed, instead of the plurality of SiO2 islands, below the SiO2 film.
A method for manufacturing the SOI substrate according to the present invention in the first aspect is a method for manufacturing the SOI substrate of the present invention including the steps of implanting oxygen ions in a silicon substrate, raising the ambient temperature of the silicon substrate wherein the temperature is raised at a rate not higher than 1xc2x0 C./minute over a range of at least 100xc2x0 C. above a temperature higher than 600xc2x0 C. while the oxygen concentration in the atmosphere is kept at a level of 0.5% or higher, and performing a heat treatment where the silicon substrate is kept at a temperature not lower than 1200xc2x0 C. for a predetermined period of time.
A method for manufacturing the SOI substrate according to the present invention in the second aspect includes the steps of:
implanting oxygen ions in a silicon substrate to form a damage peak where crystal defects generated by the ion implantation is maximum and a density peak where a concentration of the implanted oxygen ions is maximum in the silicon substrate; and
forming an SiO2 film extending in a direction parallel to a main surface of the silicon substrate and having a center of the SiO2 film located substantially at the damage peak.
According to the present invention, since the crystal defects generated in the Si active layer can be reduced when the SOI structure is formed by a low-dose SIMOX process, the SOI substrate having a satisfactory active layer can be obtained by the low-cost, low-dose SIMOX process.
The SOI substrate of the present invention can be formed by the SOI substrate manufacturing method of the present invention, and has the SiO2 film which continues in the direction parallel to the substrate surface with the center of the film located at the damage peak where the crystal defects (damage) generated during the oxygen ion implantation are maximum. With such a constitution being employed, since the residual crystal defects are mostly located below the SiO2 film and are isolated by the SiO2 film from the Si active layer, the Si active layer having less crystal defects can be obtained. In this case, a plurality of SiO2 islands are formed around the position of the density peak of the oxygen concentration, as the center thereof, below the SiO2 film during the heat treatment. Depending on the heat treatment condition, or by applying an additional heat treatment, the SiO2 islands may substantially disappear or may be turned into a second SiO2 film.